Heat treatment of magnetic material



Jan. 6', 1931. ca.v w. ELMEN '1,788,017

HEAT TREATMENT OF MAGNETIC MATERIAL f Filed July 6'. 1925 Panarea Jan. e, 1931 UNITED S'IAllss PATsNr omer.

GUSTA! W. lEIIIMEN, OF'LEONIA, NEW JERSEY, ASSIGNOB TO 'WESTERN m0 OOI'-, Pm; INCORPORATED, OF NEW- YOBK, N. Y.,A CORPORATION F YORK l HEAT TREATMENT 0I' MAGNETIQ. MATERIAL A' Applicbtion med Italy 6,

This invention relates to an improved method of producing magnetic materials and especially alloys having high permeability at low magnetizing forces, and a low hysteresis g loss. By low magnetizing forces is meant m gree, of a class ofi-magnetic materials con- IB ties. Reference may also' be had to an ar ticle by Arnold and Elmen, published in the l sisting chiefly of nickel and iron.-

. In patent to G. W. Elmen, 1,586,884, June 1, 1926, there is described a proper treatment of these alloys to obtain the desired proper- Journal of the Franklin Institute, volume 195, No. 5, May 1923, entitled Permalloy. The usual method of treating nickel-iron alloys as set forth in the prior Elmen patent and in the paper by Arnold and Elmen, comprises casting the alloy, alternatelyy working and annealing it until the desired shape or form is obtained, and then heat-treating it in its final form. The heat treatment consists of heating the material to a temperature of at least 900 C. for some time, cooling to a temperature somewhat above the transition temperature at about the usual rate employed in annealing, then cooling at a much more rapid rate to a temperature of about 300 C.

The proper rapidl rate of cooling` 'can be determined by trial in each case. The rate will, of course, depend somewhat upon the size and form of the material as well as-the composition.l One form of the material which has been found convenient vfor a variety of uses is ribbon or tape about .005" or .006 in thickness, formed into a loosely wound coil. In` a particular sample of this tape, the proper rate of cooling from-600 C. to 300 C. was found to be about 15y C. per second for about 20 seconds. The proper rate in all cases where rapid cooling is beneficial appears to be one between that which would ordinarily be used to anncal and one whichv would set up internal strains, due to premature hardening of the'exterior portions. The rapid cooling is of most benefit when the ratio of nickel to iron isaround 4.' As the nickel content is f ature.

19:5. serial no. 41,490.

decreasedthe-advanta of ra id over slow coohng decreases untiit near y disa when the ratio becomes about 1. It a decreases somewhat when the ratio is raised to 8 or 9.- Whenother elements are added the percentage of iron and nickel for which the rapid cooling is beneficial varies considerably and depends largely upon the percentag? of the added element. For example,- w 11e 781/2% nickel, 191/2% iron and 2% highest. permeability if the chromium content 1s increased to about 8% or even somewhat less, rapid cooling gives no better results than slow cooling.

0 Many. of the alloys containingnickel and iron are'sensitive to strain-the permeabilit decreasing markedly when stress is applied It is for this reason that heat treatment is employed after the4 material is in its nal form when these alloys are used.

The primary objects of this invention are to insure a uniformly high value of desired properties of the various alloys containing iron and nickel over the range from` about 25% or 30% of nickel upward, and to provide a convenient manner of performing the chromium requires the rapld coollng for'.

various steps of the heat treatment when the cordance wit one feature of the invention,

the material'is subjected, during the initial t stage of the `heat treatment, to a temperature which will produce homogeneity of the m-aterial. In'general this temperature is in the region of -1100 C., buta higher temperature is not ordinarily 'detrimental and may even be beneficial.

In accordance with another. feature of the invention, the heattreatment above outlined is performed in two stages to accurately and conveniently control the temperature of the material and the rate of cooling.

In ,heA accompanying drawings, Fig. 1 is a cross-section of a furnace suitable for heating the material. Fig. 2 is a graph showing initial permeability plotted against temper,-

'Ironand nickel (and other elements if any are present) are fused together in an induction furnace. For example, the material may be nickel and iron alone in theproportions V2li/2% of iron and 'TS1/2% of nickel. Good commercial grades of these two materials are suitable for `this purpose. The molten composition is ppured into a mold and cooled to'form a'thic the alloys are to be used for the pur ose of loadin signaling conductors, this ar is trea bythe method outlined in the patent to Elmen referred to above to produce a tape of .006" thickness and .125"-w1dth. .In treating the alloy in accordance with this invention, the tape may be'formed in aloose coiland annealed by packing in ordinary castiron or nichrome boxes 10, Fig. 1, called annealing pots, each provided with a cover 11 luted with ire-clay,'or the cover may fit inside the pot with a space above it which is filled with iron wder, finely divided copr or other rea ily oxidizable material 12. inely divided .co per is-sometimes placed within the pot. ese precautions are taken in order to prevent the free access of air to the inside of the pot duringthe annealing process and to eliminate the oxygen by means of the iron, copper or-other material, thus leaving only nitrogen and preventing oxidation of the material under treatment. The boxes or annealing pots containing the material are heated to about 1100 C. and kept at that temperature until the alloy has reached the equilibrium stage for that temperature, that is. until uniform structure is obtained throughout the material. It has been found that for these alloys this uniformity of structure is beneficial tothe magnetic characteristics. Ordinarily, this homogeneity is obtained by heating the pots for one hour after reaching the desired temperature. This heating may be done in any suitable furnace such. for example, as the furnace 13.

When the material is initially heated to 900 C., the lower limit of the range given in the abovementioned Elmen patent and the only value there specifically given, the permeability ishigh and the other desirable properties are obtained as set forth in that application. above that value. say to 1100 C.. a. somewhat higher permeability is ordinarily obtained. Moreover, the lower temperature does not give. quite uniform results with various samples of materiahwhile uniformity is obtained, by the use ofthe higher temperature. `In

order to obtain the best results the pots are left in the furnace after heating Iand allowed to cool down to room temperature with the furnace, orthey may be removedl and allowed to cool at a somewhat more rapid rate in the air. The cooling of the material, however, will be no more rapid than the coolin of the heavy annealing pots and will orinarily give a material havingA a. permeability corre# spending to the lower curve B shown in Fig.

rod or bar.' v When By raising the temperaturey :maar:

2. The annealin has also in someA cases been done in vacuum urnaces. In that case there is no need for any additional precautions to be taken in order to prevent oxidation or other impurities lfrom contaminating the samples.' This method 'may also be of advantage in 'case it is desired to do the 'annealing under low air' pressure or in the presence of other gases. 1

In a secondstage of the process which is used when' rapid coolingois advantageous the material is removed pot and heated up in the oven to a tem erature above the transition point. This will be in the neighborhood of 600 C. for the sample referred to above, but will be dependent'u' on the alloy -being treated. The materiaFis then removed and may be placed on a copper plate in the air at room temperature and allowed to cool. This relativelyrapid cool` ing-in this ran e insures that'the permeability will fol ow the upper-"curve fA. of

loys become less sensitive to strain as the nickel content is decreased fromabout 78% or 79%, the initial permeability decreases and the resistivity increases.l "In the region of 80% or 81% nickel'and somewhat above, the sensitivity to strain also falls off rapidly. See U. S. patent of O. E. Buckley, No. 1,695,038, granted December 11, 1928.' It has been discovered, however, that in all these alloys in the range from about 25% or' 30% nickel upward, very high initial perl meability at magnetizing forces of the order used in signalin (afe'w tenths of a auss or less) may be o tained. Other ingredlents,` such as chromium for example, may be added t these alloys without destroying the property of high initial permeability at small ma netizing forces. ot only do these alloys have very high permeability at low magnetizing forces when treated in the manner described but they also have a much lower hysteresis loss than iron. After cooling, if the material is in the'forrn of tape and is tobe used for loading signaling conductors, it is given va certain degree of hardness by working or otherwise, after' m the annealing 'los iis

ian

iso

' ductor, eac

ing of the conductor is desired, two or more layers ofta ma be wrapped upon the conpre erably inthe opposite 'direction from the last in order to produce the most uniform construction. If the alloy is to be used for cores in loading and filter coils,

lthe tape may be wound to a suflicicnt number y of turns to give the coil the required crossl section and then impre ated with bakelite varnish in the manner escribed in the patent to G. W. Elmen, 1,586,889, June 1, 1926.

Other uses of permalloy. and other forms than tape obviously come within the spirit and scope of the invention. It is also obvious that other forms and types Aof oven may be used in carrying out the rocess and other methods of heating and coo ing may be em'lo ed. For instance, if the material is suc t at rapid cooling is beneficial and the principal Objectis to .obtain the advantage accruing from heating the permalloy to 1100 C. as outlined above, .the material after being1 heated to thisl temperature may be cooled s owly to a temperature near 600 C., the furnace opened and the material removed directly to the copper plate and allowed to cool rapidly, vvrather than allowing the material to '1 coo to room temperature slowly in the oven and. heating up again to @gi C. before allowing theparts to cool rap- 1 y.

What is claimed isi* y 1. The method of increasing theperineability, at magnetizing forces of less than .2

gauss, lof a magnetic alloy lconsisting chiefly ofnickel and iron, which comprises heating the material to at least 1100 C. but below the melting point of the alloy and cooling it at an annealing rate of cooling.

2. The method of increasing the permeability, at magnetizing forces of less than .2 gauss, of a magnetic alloy consisting chiefly of nickel and iron, and in which the nickel content is at least 45% of the whole, which comprises heating the valloy to a temperature of at least 1100 C., but less than the fusing temperature of the alloy, and cooling it at an annealing rate of cooling.

3. The method of heat treating a magnetic alloy consisting chiefly of. nickel and iron and in which the nickel content is about 78% to 80% of the whole, which comprises heating the material to about 1100 C. and cooling at an annealing rate of cooling.

4. The method of increasing the permeability, at magnetizing forces of less than .2 gauss, of a magnetic alloy consisting chiefly of nickel and iron and in which the nickel component comprises at least 45% of the whole, which comprises heating said material until its structure is uniform throughout,A

rate in excess of anannealing rate but notl so rapid as to setup strains in the'material due to uneven hardening.

5. A method in accordance with claim 4 in which the first heating to unify the structure of the material is to at least 1100 C. but less than the melting point of the alloy.

'6. The method of increasing the permeability ot magnetic alloys at magnetizing forces less than .2 gauss which com lrises rst heating to a temperature 'of abou 1100 C. and cooling the material through a considerable range of temperature at an annealing rate and second,- bringing the material to a temperature above the magnetic transition temperature of the alloy and then cooling through and somewhat below the magnetic transition temperature as rapidly as can be done without setting up stresses in the cooled alloy due to uneven hardening.

7. The' method of increasing the permeability'of magnetic alloys consisting chiefly of nickel and iron which comprises first heating to'about 1100 C. and vcooling at an' annealng rate. and second, reheating to a temperature slightly above the magnetic transition temperature, and cooling at a rate in excess of an annealing rate but less than that at which strains result from uneven hardening. y

8. The method of increasing the permeabilitv of a magnetic .alloy consisting chiefly of nickel and iron and in which the nickel component is at least about 45% ofthe whole, which comprises heating the material to about 1100 C., cooling at an annealing rate to a point at least 100 C. below the magnetic transition temperature, reheating a temperature in the vicinity of the transition temperature of the alloy, and cooling at a rate more. rapid than the first cooling but not rapid enough to setup stresses -in thecooled material dueto uneven hardening.

9. The method of improving the magnetic properties of a magnetic composition consisting chiefiy of nickel and iron? in'whch the nickel' component is at lea'st 45% of the iron- '.nickel content, which. comprises heating the material to about 1100 C., cooling at an annealing rate to aboutthe magnetic transition temperature, and continuing the-cooling to room temperature at' a 'rate more rapid than 'the first cooling but not rapid enough to set up stresses in the cooled material due to uneven hardening.

In witness whereof, I hereunto subscribe my name this 30th day of June, A. D. 1925. GUSTAF W. ELllirEN.

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